Water-based inks are being widely used as inks for printers and the like in consideration of the environment and include: dye inks in which a dye serving as a colorant is dissolved in an aqueous medium; and pigment inks in which a pigment serving as a colorant is finely dispersed in an aqueous medium. Dye inks have problems of light resistance and water resistance and, therefore, pigment inks are advantageously used in applications requiring light resistance and water resistance. However, in the case of using a pigment as a colorant, a failure of dispersion of the pigment occurs unless pigment particles are dispersed as fine particles in a medium and stabilized in that state. If the pigment with a dispersion failure is used as an ink, the failure emerges as lacks of ink characteristics, such as a decrease in coloring power, clogging of pores or deterioration in storage stability due to increased viscosity. Therefore, the pigment is required to have dispersion stability.
Meanwhile, because of high fastness of prints using pigment inks, there is a growing demand for pigment inks in the field of photographic printing requiring high print density (color developability). Therefore, printing on a recording medium having low pigment ink permeability, such as glossy paper, is being required. However, because in this case the pigment does not penetrate into but stays on the surface of the recording medium, there arises a problem, for example, in that when an image-formed portion is in friction with something, the pigment is peeled off from the surface, so that the image is diluted or erased. This phenomenon is referred to as a fixation failure. As a solution to improve fixability, there is known a technique for adding, in addition to a pigment dispersant, a water-soluble resin or a resin emulsion to the ink. However, in terms of the composition design of ink, there is a need for a pigment dispersant having both pigment fixation stability and pigment dispersion stability.
Patent Literatures 1 and 2 propose to use, as a pigment dispersant made of a block copolymer, an A-B diblock copolymer made of a hydrophilic A block and a hydrophobic B block. Patent Literature 3 proposes to use an A-B diblock copolymer made of a hydrophilic A block and a hydrophobic B block or an A-B-C triblock copolymer made of hydrophilic A and C blocks and a hydrophobic B block.